The present invention relates to liquid crystal displays nd more particularly to a dual cell, transflective liquid display in which the contrast ratio in the transmissive mode is at least equal to and possibly better than it is in the reflective mode.
Liquid crystal devices are widely used in a variety of display applications. One of the principal reasons for the popularity of liquid crystal displays is their small size and low power consumption. Liquid crystal cells of the guest-host variety are particularly popular because they do not require polarizers; they have color capabilities; and they are intrinsically brighter than those utilizing polarizers.
One of the most significant forms of guest-host liquid crystal cells utilizes a pleochroic guest dye which exhibits anisotropic optical behavior; i.e., it absorbs light along one axis (the long axis) and transmits light along the other axis. The liquid crystal host contains an optically active material for imparting a helical order to the liquid crystal molecules so that the dye molecules spontaneously align themselves with the helically ordered liquid crystal molecules. In the absence of an electric field, with the dye molecules aligned in a helical order, (i.e., with their long axes parallel to the cell electrodes) they absorb light. When an electric field is applied to any portion of the cell, the helically ordered liquid crystal molecules in that portion unwind, i.e., the molecules assume a homeotropic order (with the nematic directors of the molecules at right angles to the cell electrodes). The long or light absorbing axes of the dye molecules also assume a homeotropic order and are thus oriented to transmit light through the energized portions of the cell.
The cell contrast ratio, i.e., the contrast between the cell display elements and the unenergized cell background portions, is an important characteristic of any cell because the contrast ratio affects the overall visibility and appearance of the display.
Control of contrast ratio is of particular significance in transflective liquid crystal displays because the contrast ratio can vary in such cells depending on the operational mode. The term "transflective" liquid crystal device is used broadly to denote an arrangement in which a liquid crystal cell may be operated in a reflective mode, a transmissive mode, or simultaneously in both modes. In the reflective mode, light (whether ambient or from a specific source) enters the front of the cell, passes through the guest-host solution to a transflective element at the back of the cell which reflects 80-95% of the impinging light. The reflected light passes back through the solution to the front of the cell.
In the transmissive mode, on the other hand, light from a source at the rear of the cell illuminates the transflective element which passes a portion of that light through the liquid crystal guest-host solution to the front of the cell. In a single cell display operated in the transmissive mode, it can be seen that light passes through the solution only once, while it passes through twice in the reflective mode. Light absorption by the dye in the unenergized background portions, is therefore much lower in the transmissive mode and the contrast ratio decreases markedly in this mode.
It has been suggested that the effect of differing contrast ratios on the overall appearance of a single cell transflective display might overcome by increase the cell thickness or the dye concentration or both. The premise being that if this increases the contrast ratios sufficiently, the ratios, though not the same in both modes, are sufficiently high in both modes (viz, 100 to 1 in the reflective and 25 to 1 in the transmissive) to be adequate. It has been found, however, that this had a deleterious effect on the brightness of the cell in the reflective mode.
Applicant has found that the contrast ratio of a liquid crystal display in the backlighted or transmissive mode may be enhanced to maintain the contrast ratio at least equal to that in the reflective mode without any deleterious effect on brightness by providing a cascaded, dual cell arrangement.
The second cell is positioned behind the transflective element. The spacing and dye concentration of the two cells are preferably the same. The display elements in the second cell are aligned with those of the first cell; and the two cells are both energized simultaneously in the transmissive mode.
In the transmissive mode, light passes through both cells and the contrast ratios controlled so that they are at least equal in both modes. In this fashion, the front cell thickness and dye concentration can be such as to optimize brightness while enhancing the contrast ratio in the transmissive mode by adding the second cell.